While Valens persecutes the Orthodox Christiansin the East, a Usurper arises at Constantinople named Procopius: and at the Same Time an Earthquake and Inundation take Place and injure Several Cities
While Valens was thus occupied in Syria, there arose a usurper at Constantinople named Procopius; who having collected a large body of troops in a very short time, meditated an expedition against the emperor. This intelligence created extreme solicitude in the emperor's mind and checked for a while the persecution he had commenced against all who dared to differ from him in opinion. And while the commotions of a civil war were painfully anticipated, an earthquake occurred which did much damage to many cities. The sea also changed its accustomed boundaries, and overflowed to such an extent in some places, that vessels might sail where roads had previously existed; and it retired so much from other places, that the ground became dry. These events happened in the first consulate of the two emperors.
Source: Ecclesiastical History (New Advent)